The present invention relates generally to magnetic resonance imaging systems and more particularly to a coolant-cooled RF body coil for use in a magnetic resonance imaging system.
Magnetic Resonance Imaging (MRI) is a well-known medical procedure for obtaining detailed, one, two and three-dimensional images of patients, using the methodology of nuclear magnetic resonance (NMR). MRI is well suited to the visualization of soft tissues and is primarily used for diagnosing disease pathologies and internal injuries.
Typical MRI systems include a superconducting magnet capable of producing a strong, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including transmitter and receiver coils, also surrounding or impinging upon a portion of the patient; a magnetic gradient coil system also surrounding a portion of the patient; and a computer processing/imaging system, receiving the signals from the receiver coil and processing the signals into interpretable data, such as visual images.
The superconducting magnet is used in conjunction with a magnetic gradient coil assembly, which is temporally pulsed to generate a sequence of controlled gradients in the main magnetic field during a MRI data gathering sequence. In as much as the main superconducting magnet produces a homogeneous field, no spatial property varies from location to location within the space bathed by such field; therefore, no spatial information, particularly pertaining to an image, can be extracted there from, except by the introduction of ancillary means for causing spatial (and temporal) variations in the field strength. The above-mentioned gradient coil assembly fulfills this function; and it is by this means of manipulating the gradient fields that spatial information is typically encoded.
The actual image data consist of radio frequency signals, which are stimulated and received by means of systems of resonant radio frequency coils that irradiate the patient in the scanning volume. These coils typically fall into two classes: volume and surface resonators.
A magnetic resonance apparatus has a plurality of gradient coils that are arranged on a (superconductive) magnet, such as the exterior of the cryostat. These gradient coils each generate a magnetic field with a linear gradient, which is essential for generating image signals. Three gradient coils are normally provided, respectively generating linear magnetic field gradients during operation of the magnetic resonance apparatus that are directed perpendicular to one another. The directions of these gradients are usually indicated as the x, y and z-axis of a Cartesian coordinate system.
A diagnostic nuclear magnetic resonance device of the above general type is known from U.S. Pat. No. 4,954,781. The nuclear magnetic resonance device disclosed therein has a cylindrical examination chamber that accepts a patient to be examined. The examination chamber is surrounded by a superconducting magnet that generates a homogeneous main magnetic field in the examination chamber extending in an axial direction, i.e. the z-direction. A cylindrical carrier tube is arranged between the superconducting magnet and the examination chamber, to which gradient coils are attached for the generation of gradient fields in directions perpendicular to one another, of which one direction coincides with the direction of the main magnetic field in the z-direction. High-frequency antennas are likewise fastened to the carrier tube, by means of which nuclear spins in an examination subject are excited and the resulting nuclear magnetic resonance signals are received.
In the operation of the nuclear magnetic resonance apparatus for the generation of sectional images, the gradient fields must be switched on and off. This is achieved by supplying the gradient coils with switched currents of different amplitudes and different switching frequencies, with the direction of the currents through the gradient coils additionally being changed. This has the consequence that the conductors of the gradient coils, as well as the carrier tube, heat up. Furthermore, the conductors are exposed to oscillating forces that produce bothersome noises. In modern imaging sequences, particularly during rapid imaging, the gradient coils can reach high temperatures and can emit high acoustic noise levels.
The present invention addresses these concerns by providing a thermal barrier between the patient bore and each of the gradient coil assembly and RF body coil assembly to maintain the temperature within the patient bore below a maximum operating temperature. This allows the RF body coils to run cooler and also provide a barrier to the gradient coils.
In one preferred embodiment, a hollow conductor structure is introduced to the RF body coils at a position between the gradient coils and the patient bore tube. Cooling water, or other coolant, introduced through the hollow conductor structure captures heat generated from the gradient coils and RF body coils during MRI scans, therein preventing the heat from entering the patient bore.
In another preferred embodiment, the hollow conductor structure replaces the flat copper strip of the prior art and itself functions as the RF body coils. Again, as in the other preferred embodiment, cooling water introduced through the hollow conductor structure captures heat generated from the gradient coils and RF body coils during MRI scans, therein preventing the heat from entering the patient bore.
The present invention thus allows the RF body coils to run cooler and provide a thermal barrier to the heat emitted by the gradient coil during MRI scans. Therefore, the patient bore of the system will be cooler during the scans. This in turn allows the scans to be longer without adversely affecting the patient. Another potential benefit is that the hollow conductor will provide a stiffer RF body coil, which may reduce acoustical noise generated during scanning operations.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.